1. Field of the Invention
The present invention relates to a system and method, for use with a network, such as a satellite-based communications network, which establishes an integrated forward error correction (FEC) scheme to perform multi-rate encoding/decoding on different priority data bits transmitted, for example, on a random access channel. More particularly, the present invention relates to a system and method for using a single encoder to perform multi-rate encoding/decoding on different priority data bits of a channel request message transmitted on a random access channel from an access terminal to a base station of a satellite-based communications network.
2. Description of the Related Art
A satellite-based communications network, such as a geosynchronous earth orbit mobile (GEM) satellite communications network, comprises at least one geosynchronous earth orbit satellite, a ground-based advanced operations center (AOC) and spacecraft operations center (SOC) associated with the satellite, at least one ground-based gateway station (GS), and at least one access terminal (AT), which is typically a hand-held or vehicle mounted mobile telephone. The satellite enables the access terminal to communicate with other access terminals, or with other telephones in the terrestrial public switched telephone network (PSTN), via the gateway stations under the control of the gateway stations. The AOC provides system-wide resource management and control functions for its respective satellite, and the SOC controls on-orbit satellite operations for its respective satellite.
When an access terminal is operated to establish a radio resource connection, it generates and transmits a channel request message to the network on a random access channel (RACH) at a frequency assigned by the gateway station to a spot beam covering an area in which the access terminal is located. A channel request message includes data used to represent the mobile access terminal, as well as contention resolution and timing synchronization information.
Typically, the data bits of a channel request message are classified into two classes, namely, Class I bits and Class II bits. The Class I bits generally consists of a short block of data bits, and represent high priority information, such as data for contention resolution and timing synchronization between the access terminal and the base station, which is essential in enabling the access terminal to gain access to the satellite communications network. Therefore, the Class I data bits must be received with high probability by the base station the first time they are transmitted.
On the other hand, the Class II bits typically consist of a longer block of data bits, and are not as crucial as the Class I bits for call set up. Class II bits thus have a lower priority than the Class I bits. Class II bits can include, for example, data for accelerating call set up time, which can include information such as the called party number, location of the access terminal placing the call, identification of the service provider for the access terminal, and so on.
During transmission of a channel request message, an access terminal will encode the Class I and Class II bits to increase the probability that they will be received intact by the base station. Class II bits can be encoded using, for example, a GEM baseline rate xc2xd convolutional code. However, because the Class I bits are necessary for call set up and have a higher priority than the Class II bits, the access terminal will encode the Class I bits at a higher rate convolutional code.
Because the block of Class I bits generally has a short length, it is difficult to convolutionally encode the block of Class I bits by itself. Therefore, to encode the Class I bits, an access terminal can include a block code encoder in addition to the convolutional encoder used to encode the Class II bits. In this event, the base station receiving the encoded channel request message would require a block code decoder to decode the encoded Class I bits in addition to a convolutional decoder for decoding the Class II bits. These additional block code encoders and decoders in the access terminal and base station increase the overall complexity of the system, which can result in additional cost of the system as well.
Accordingly, a need exist for a satellite-based communications network capable of encoding and decoding data bits of different priorities in a channel request message at different coding rates without using additional encoders and decoders.
An object of the present invention is to provide a system and method for establishing an integrated forward error correction (FEC) scheme to perform multi-rate encoding/decoding on different priority data bits of a channel request message transmitted over a random access channel in a satellite-based communications network, without using multiple encoders and multiple decoders.
Another object of the invention is to provide a system and method which uses an encoder having a single coding rate to encode different priority bits of a channel request message transmitted over a random access channel in a mobile satellite communications network at different coding rates which are dependent on the respective priorities of the data bits.
A further object of the invention is to provide a system and method which uses a single encoder in an access terminal of a satellite-based communications network to encode different priority data bits of a channel request message at different respective encoding rates, and which uses a single decoder in a base station of the communications network to decode the differently encoded bits of the channel request message received from the access terminal over a random access channel.
These and other objects of the invention are substantially achieved by providing a system and method for encoding data, including a first data group representing first information and a second data group representing second information, which is transmitted between an access terminal and a base station in a satellite-based communications network. The system and method encodes the second data group at an encoding rate to provide a second encoded data group, and encodes the first data group at the same encoding rate to provide a first encoded data group. The encoding of the first and second data groups is performed by a single encoder, such as a rate xc2xc convolutional encoder. The second encoded data group is transmitted between the access terminal and the base station or, in particular, from the access terminal to the base station over a random access channel, for example. The second encoded data group further can be punctured during transmission to in effect decrease its coding rate, for example, to rate xc2xd coding. The first encoded data group is transmitted from the access terminal to the base station, and is then retransmitted from the access terminal to the base station to in effect increase the rate of coding of the first encoded data group to, for example, xe2x85x9 coding. As can be appreciated by one skilled in the art, the transmission of the second encoded data group, and the transmission and retransmission of the first encoded data group, occurs within a single burst. At the base station, a combiner/demodulator combines the transmitted and retransmitted first encoded data group, and the combined first encoded data groups and the second encoded data group are then decoded by a decoder at the base station.